1. Field of Invention
The invention relates to a white-light LED with dielectric omni-directional reflectors used in a light-emitting module that emits white light.
2. Related Art
The light emitted from normal home illuminators such as lamps, U-shape bulbs, flashlights, and illuminators inside vehicles/airplanes/ships is a white light with three wavelengths. The backlit source used in the current TFT-LCD""s is also a white light with three wavelengths. It is thus seen that the light-emitting modules for producing white light occupy a great portion of the illumination market.
An omnidirectional dielectric reflector is a multi-layer of transmitting materials in a stacked structure with feature sizes on the order of a wavelength or subwavelength. It is a one-dimensional photonic bandgap crystal that exhibits strong reflection at a certain range of incident light wavelengths (stop bandwidth) irrespective of their directions of propagation and electric field polarizations. In other words, this type of material structure is said to posses a complete photonic bandgap. This leads to the possibility to control the spontaneous emission of an LED. The so-called white light LED is composed of a light-emitting diode (LED) and a phosphor grains layer. The white light can be generated in the phosphor grains layer when light emission from excitation source LED are absorbed and converted into fluorescence. Each color of visible light can be generated from suitable phosphors by using blue, violet and ultraviolet excitation light, then with the combination of converted fluorescence from phosphor grains layer and/or light from LED can generate white light emission. This white light-emission device has few technique difficulties: (1) control of the phosphors white light conversion efficiency. (2) Control of the unpolarized light emission from the uv or blue LED with isotropic angular distribution. (3) Uniformity of fluorescence intensity distribution (4) regulation of the correlated color temperature. Moreover, the so-called xe2x80x9cwhite lightxe2x80x9d in this specification refers to a mixture of light with several colors. The usual white light observed by human eyes comprises at least two colors of light with different wavelength ranges which are sensitive to human eye. The three color luminous intensity ratio of output fluorescence generally required blue 5-25%, green 20-50% and red 40-80% in order to have a desired color temperature, and the color coordinates are in the range of x=0.22xcx9c0.4 and y=0.22xcx9c0.4. For example, U.S. Pat. NO. 6,084,250 discloses the control of color rendition by the composition of three phosphors. One may also combine a white light with two different wavelengths can be obtained by mixing blue light and yellow light. Therefore, the enhancement and/or control of conversion efficiency for phosphor grains layer can be significant to solve the above-mentioned technology difficulties.
The white-light LED can be classified according to the material filled inside as organic and inorganic ones. A commercially mature product is an inorganic white-light LED developed by Nichia Chemical, Inc. A schematic view of its structure is shown in FIG. 1. Surrounding the blue LED chip 10 is filled with yellow-light phosphor grains 20. The wavelength of the blue light emitted by the blue LED chip 10 is between 430 nm and 480 nm. Using the light emitted by the blue LED chip 10 to excite the yellow-light phosphor grains 20 will produce some blue light at the same time the yellow light is produced. The combination of the blue and yellow light provides a white light with two wavelengths.
However, since the blue light occupies a great portion of output luminance produced by the white-light LED consisted of the blue LED chip 10 and yellow-light phosphor grains 20, the color temperature tends to be higher. Moreover, the color temperature of this type of white light devices is hard to control. Therefore, one has to increase the chances for the blue light to interact with the yellow-light phosphor grains 20, in order to reduce the luminous intensity of the blue light and/or enhancing that of the yellow light.
To solve the above-mentioned problems, the U.S. Pat. No. 5,962,971 discloses an LED that uses an UV filter layer 30 coated on the top of the phosphor grain layer 40. This means makes the phosphor grains layer 40 emits light with a greater homogeneity. It also filters out the UV light emitted by the LED chip 50 to avoid damages to human eyes. As a consequence, the UV light is unnecessarily wasted, lowering the conversion efficiency of the phosphor grain layer 40.
The U.S. Pat. No. 5,813,753 discloses a UV/blue LED-phosphor device, where a short wave pass filter is coated on the light-emitting surface of the UV/blue LED stack. The functions of the short wave pass filter are: (1) to reflect light of the too long wavelength and (2) to reflect part of the light of the wanted wavelengths. The overall result is a more narrow angular distribution in the forward direction, and furthermore a more saturated color. On the other hand, on the outgoing surface of the UV/blue LED and phosphor grains layer structure, a long wave pass filter is coated to enhance the transmission of the visible light, and to reflect UV/blue light back to the phosphor grains layer. However, the conversion efficiency and LED light traveling path is not properly controlled because of the unpolarized and isotropic angular emission of the light from the UV/blue LED chip. In other words, dielectric omni-directional reflectors for the light from the UV/blue LED chip is need for enhancing the conversion efficiency.
In view of the foregoing, an objective of the invention is to provide a white-light LED with dielectric omni-directional reflectors. Dielectric omni-directional reflectors are functionally used to replace the long-wave pass filter (LWP) and/or UV/visible mirror as stated in U.S. Pat. No. 5,813,753 and coated on both side of the phosphor grains layer. Both sides of the phosphor grains layer form a Fabry-Perot like resonance cavity to enhance the light emission efficiency of the white-light LED. These dielectric omni-directional reflectors only reflect light of a specific wavelength range irrespective polarization and incident angle, such as a blue light with a wavelength of 450-500 nm or an UV light with a wavelength of 380xcx9c400 nm.
The invention related to a white-light LED with omni-directional reflectors includes an LED chip for an excitation light source. A light transmitting material surrounding the LED and phosphor grains is dispersed in order to excite fluorescence via emission of LED. The visible-light spectrum of the phosphor grains has to be compatible with the emission wavelength of the LED. And at least one or two omni-directional reflectors are symmetrically placed on the top and/or bottom of the structure including the LED chip and the light transmitting material surrounding the LED chip. When the LED chip emits UV or blue light that passes through the light transmitting material, the phosphor grains inside the light transmitting material are excited to produce secondary visible lightxe2x80x94the fluorescence.
Since the dielectric omni-directional reflectors surrounding the light transmitting material will reflect UV or blue light repeatedly, the light from the LED is reflected omni-directionally via the dielectric omni-directional reflectors. This forms an omni-directional Fabry-Perot like light resonance cavity, i.e. the excitation light is confined in the phosphor grains layer. By reflecting the UV or blue light multiple times between the dielectric omni-directional reflectors, the phosphor grains can be excited repeatedly to increase the white-light conversion efficiency and/or to change the spectral characteristics of the visible light emission. Thus, the disclosed white-light LED can emit more white light, and/or the correlated color temperature can be changed.
As the dielectric omni-directional reflectors were designed not to totally reflect the blue light emission produced by the light emission diodes, and the blue light can be partially transmitted through the dielectric omni-directional reflectors and can be observed. Moreover the transmittance of the dielectric omni-directional reflectors for the blue light is designed so that the correlated color temperature of the white light LED can be controlled.